If an employee deletes their invitation, will they get a new one?

Yes. Deleting an invitation or a reminder doesn't stop the reminders. Retain keeps sending them on its normal schedule, each with a survey link.

It also doesn't change the survey itself. The open survey stays open, and the employee can still take it, from a reminder link or from their profile.

The one exception is if they marked a Retain message as spam or junk, which stops future messages.